The invention relates to a hydraulically-operating shock and vibration absorber or damper, having a cylinder closed at one end, and a piston which is slidable within the cylinder. The piston is provided with a port and is connected with a hollow piston rod that extends out of the open end of the cylinder. The interior of the cylinder and the piston rod define a chamber filled with a incompressible damping fluid. A space is defined in the piston rod separated from said chamber by a slidable separating piston. The space contains a compressible agent. The design of the shock absorber is such that when it is under load the damping fluid can be pressed out of one portion of the chamber, through the port, into the another portion. Similarly, when the load is removed, the damping fluid is returned to the first portion through the port.
In the issue of the U.S. journal "Machine Design" dated 24th May, 1962, there is described a series of hydraulically-operating shock absorbers with jet orifices which change under the load of impact. The idea of these shock absorbers is to obtain a reduction in the kinetic energy of the impact which is as uniform as possible along the length of the stroke. In these known shock absorbers, oil is used as hydraulic liquid. In this literature source, various types of jet orifices are described. These serve to convert part of the impact or kinetic energy into heat and at the same time compress a compressible damping medium.
Jet orifices whose diameter is varied by means of a metering pin are used as control parts in these apparatuses. Alternatively, control grooves of varying diameter in the inside wall of the cylinder, past which the piston is pushed are used. These known shock absorbers have the disadvantage, however, that for a given opening using oil as a hydraulic liquid (Newtonian fluid), only a very small buildup of damping force can be obtained over the length of the stroke or travel; if the jet orifice is too big, the oil does not undergo the necessary throttling effect and thus cannot build up the necessary pressure difference. Only with a correspondingly small jet orifice can a greater damping force be obtained. Moreover, with a large nozzle diameter and thus lower speeds, the proportion of laminar flow through the orifice is much greater. This results in there being a strong dependence of the oil hydraulic damping action on the temperature of the oil.
The known shock absorbers are therefore not universally applicable. They are only suitable for damping a very definite shock load which is always of the same magnitude, provided that the damper is constructed in accordance with the respective need.
U.S. Pat. No. 2,963,175 relates to a hydraulically-operating shock absorber for railway wagons which consists of a cylinder and a hollow piston rod supported slidably in the cylinder. In the base of the piston rod there is an opening through which a metering rod extends. The metering rod is fixed at one end to the bottom of the cylinder and has at the other end a conical disc positioned in the interior of the hollow piston rod so as to prevent the rod from slipping out of the cylinder. The conical disc also functions as a flow resistor for the hydraulic oil. A complicated system of gaskets and washers, guide rings and wiping rings, positioned in corresponding grooves in the outside circumference of the hollow piston rod or in the inside wall of the cylinder is provided as a sealing means. The known damper corresponds in principle to the one known from the literature source in "Machine Design" and thus has the same basic disadvantages which have already been listed for this type of damper.
DE-AS No. 2 324 402 relates to a hydraulically-operating shock absorber consisting of a cylinder closed at one end, a piston which is slidable within the cylinder having a port and being connected with a hollow piston rod that extends out of the open end of the cylinder, a chamber filled with a non-Newtonian damping fluid, and a space, separated from said chamber by a slidable separating piston, in which there is a compressible agent. The design of the shock absorber is such that when it is under load, the damping fluid can be pressed out of one portion the chamber through the port into another portion. Similarly, when the load is removed, the damping fluid is returned to the first portion through the port.
The control port in this reference consists of a groove of varying diameter in the inside wall of the cylinder past which the piston slides. When the shock absorber is under a shock load, the non-Newtonian fluid flows out of the cylinder chamber past a collar and a piston ring into a circular groove in the lateral surface of the piston. The fluid continues from there through an opening in the circular groove into the interior of the hollow piston. As a result of the fluid flowing into the interior the separating piston is displaced. This further compresses the pressure gas in the pressure area of the hollow piston rod.
This known damper has the disadvantage of being of a very complicated construction which contains a series of precision parts which are difficult to manufacture. Additionally, the control port is formed of parts which slide past one another and are thus subject to wear there is the danger that if the damper is under continuous load, the buildup of damping force will become less due to wear and tear on the parts. Further, erosion of the control groove in the inside wall of the cylinder will in time cause separation of the piston collar from contact with it, making the damper ineffective or causing it to jam. As a damping medium, a thixotropic fluid (grease) is used. The viscosity of thixotropic fluids decreases with increasing load time for a constant load or constant shearing speed. The damping force can therefore be determined only for a certain individual load and a defined load time. Different load times would unavoidably lead to different damping force development.
U.S. Pat. No. 3,489,087 relates to a hydraulically-operating shock absorber, having a cylinder closed at one end, a piston which is slidable within the cylinder with a piston rod extending out of the open end of the cylinder, a chamber filled with a non-Newtonian incompressible damping fluid, and a space separated from said chamber by a slidable separating piston, in which there is a compressible agent. The piston has a centrally positioned through opening for the incompressible damping fluid to pass through. In the hollow piston rod, the separating piston is biased by a spiral spring against the jet orifice 18 of the piston. The jet or nozzle is very thin and long, which means that there is at least a certain amount of friction caused by the fluid passing through the opening, and thus a certain conversion of the impact energy into heat. In this device a spiral spring is used as the compressible agent; and a "pseudo-plastic" liquid, namely a hydrocarbon emulsion, polymer solution or a so-called "Bingham" liquid is used as the incompressible damping medium. This material is not defined in more detail in the reference. Ideally, the damping force when plotted against displacement should show a hysteresis loop having a rectangular form. This is not possible with the known shock absorber.
The use of a quasi permanently plastic damping medium in a damper with a simple opening and a control pin is not only not rendered obvious by the U.S. Pat. No. 3,489,087, but is even excluded. It is expressly stated in column 2, lines 37 to 54, that hydraulically-operating dampers where a metering pin is moved back and forth in a jet are not suitable for damping shock loads over a wide range because the diameter of the metering pin is a function of its length and the respective design of the metering pin is very difficult. For every damping action there is a certain pin, which, since it cannot be linear, cannot be mechanically manufactured. One is advised against the use of dampers with metering or control pins guided by a throttle opening because of the constructional difficulties involved. The design of the hollow piston rod in the damper according to the present invention differs from the space in the hollow piston rod of this U.S. patent specification where a spiral spring is used as compressible means. The reference structure can have undesirable backspringing into the starting position or a position outside the starting position after the removal of the load.
DE-OS 2 438 180 relates to an impact damper or shock absorber filled with hydraulic oil for motor vehicles. The return stroke after the impact restrained in order to prevent subsequent damage to the vehicle. This special objective is obtained by a very special operational principle. In the cylinder which is closed at one end is a slidable piston. The piston is provided with an opening and moves in the outer cylinder with enough play to allow the oil in the cylinder chamber to flow past the piston into a circular chamber surrounding the piston. The circular chamber is sealed by a bushing of Teflon or nylon. The bushing serves to brake or restrain the impact and the return stroke. This ensues in that the bushing which under normal conditions lies free of tension in the part of the outer cylinder where the diameter is increased is pressed on the inward stroke (impact) into a circular groove of the inner cylinder.
In so doing, the elastic bushing functioning as a mechanical rubber spring takes up a large part of the impact energy. After the collision, i.e. on the return stroke to the starting position, the deformed bushing decelerates the return of the cylinder by friction.